Method to fabricate copper wiring structures and structures formed thereby

ABSTRACT

Techniques formation of high purity copper (Cu)-filled lines and vias are provided. In one aspect, a method of fabricating lines and vias filled with high purity copper with is provided. The method includes the following steps. A via is etched in a dielectric. The via is lined with a diffusion barrier. A thin ruthenium (Ru) layer is conformally deposited onto the diffusion barrier. A Cu layer is deposited on the Ru layer by a sputtering process. A reflow anneal is performed to eliminate voids in the lines and vias.

BACKGROUND

1. Field of the Invention

The present invention relates to wiring structures and, moreparticularly, to techniques for the fabrication of copper (Cu)-filledwiring structures and to the improvement of their performance.

2. Description of Related Art

The deposition of copper (Cu) into the narrow copper-filled lines andvias in current high density wiring technologies is accomplished by twoquite different key copper deposition steps. The first of these steps,known as seed copper deposition step, involves the sputter deposition(PVD) of a thin layer of copper. The copper thus deposited is highlypure, but the deposition is not highly conformal and consequently, itcannot be used to deposit all of the copper required by the structure tobe formed because voids would result, especially in the vias. Tocompensate for this deficiency, after the deposition of a thin seedlayer, the remainder of the copper required is deposited using a second,very different electroplating step. While the copper formed byelectroplating is substantially less pure, the electroplating processitself is sufficiently conformal to fill the structure without voiding.

It should be noted that electroplating cannot be used alone, without theprior deposition of the seed copper layer because a seed copper layer isrequired to carry the electric current necessary to initiate theelectroplating process.

A high density wiring structure comprises lines and vias. As the fillingof the vias in the structure is more difficult than the filling oflines, in the discussion below we shall address the problem ofsuccessfully filling vias with void-free Cu. However, it should beunderstood that lines are also formed and filled by the same copperdeposition process or processes that are employed to fill the vias inthe structure. For example, in general, conventional techniques used forfabricating a Cu-filled via typically involve first forming a via holein a dielectric matrix in which the wiring structure is embedded.Second, the via hole is lined with a diffusion barrier to prevent thediffusion of Cu into the dielectric. This diffusion barrier typicallyincludes tantalum nitride (TaN) deposited directly onto the dielectric,and tantalum (Ta) deposited on top of the TaN. Third, a thin layer ofseed Cu is sputter deposited onto the exposed Ta surface, in order toprepare the via for electroplating. Fourth, an electroplating process isused to fill the via with Cu.

The fabrication of the wiring structure thus involves two quitedissimilar processes, the PVD process, performed in a high vacuumcluster tool, and the electroplating, an aqueous solution process. Itwould be desirable from a number of standpoints if the electroplatingstep could be eliminated and the PVD process could be used to depositall of the requisite copper. This approach would first reduce the costof manufacturing by obviating the need for electroplating tooling.Second, Cu electroplating baths are extremely toxic. If electroplatingcould be avoided, the expense and risks associated with handling suchmaterials could be eliminated. Third, as noted above, the copperdeposited by electroplating is less pure than that deposited by PVD.

Typical impurity levels in the electroplated Cu used to form wiringstructures are: carbon (C), 100 parts per million (ppm), chlorine (Cl),80 ppm, oxygen (O) 80 ppm, and sulfur (S), 50 ppm. These impurities alldegrade the conductivity of the copper, and chlorine can in additioncause corrosion which can subsequently lead to the failure of thestructure. By contrast, PVD deposited copper typically has impuritylevels of less than 20 ppm for each of the aforementioned elements.

In contrast to the impurities C, O, Cl, and S listed above, which aredeleterious to the performance of the wiring structure, some impuritiessuch as boron (B) and phosphorus (P) can be beneficial. In particular Band P are useful in suppressing electromigration. If all of the copperin the wiring structure is to be deposited by PVD, and it is deemeddesirable to dope the copper with beneficial impurities, it isstraightforward to do so with precise control over the concentration ofthe beneficial impurities (dopants) by the simple method ofincorporating them into the Cu sputter target in the requisiteconcentration. The processing steps described herein below areindependent of whether or not the Cu target used for the PVD processcontains deliberately introduced dopants. To simplify the discussionbelow we will take the term “pure copper” to refer to copper which isfree of deleterious impurities, such as C, O, Cl or S, with theunderstanding that “pure copper” may contain deliberately introduceddopants.

In view of the above, it is desirable to develop a technique whichprovides high purity Cu filled wiring structures using solely a highpurity PVD deposited copper process.

SUMMARY

The present invention provides techniques producing Cu filled lines andvias formed entirely from high purity PVD Cu. In one aspect of theinvention, a method of fabricating a Cu-filled via is provided. Themethod includes the following steps. A via is etched in a dielectric.The via is lined with a diffusion barrier. A thin ruthenium (Ru) layeris conformally deposited onto the diffusion barrier. This rutheniumlayer is essential for the invention. It provides a surface which thesubsequent copper layer wets, making the reflow step described beloweffective. A thick Cu layer is deposited on the Ru layer by PVD.Depending on the details of the particular structure to be filled thethick Cu layer can be of a thickness up to 200 nm, in contrast to thethin (<50 nm) Cu seed layers used in the prior art. A reflow anneal isperformed to reflow the copper in the structure to eliminate any voidsincrease a grain size of the seed Cu layer.

In another aspect of the invention, a Cu-filled via formed in adielectric is provided which includes a via, a diffusion barrier liningthe via, a thin Ru layer disposed conformally on the diffusion barrier,a think Cu layer disposed on the Ru layer by PVD and the structureannealed to reflow the Cu and eliminate any voids. In one embodiment ofthe present disclosure, the thick copper layer includes chlorine and/orsulfur each at a concentration of about 40 ppm. In yet anotherembodiment, the concentration of chlorine or sulfur is less than 20 ppm.In one non-limiting example, the copper in the structure formed therebycontains less than 40 ppm of carbon and oxygen, and less than 20 ppm ofchlorine and sulfur.

A more complete understanding of the present invention, as well asfurther features and advantages of the present invention, will beobtained by reference to the following detailed description anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional diagram illustrating a dielectric depositedover a substrate and a via etched into the dielectric according to anembodiment of the present invention;

FIG. 2 is a cross-sectional diagram illustrating the via lined with adiffusion barrier according to an embodiment of the present invention;

FIG. 3 is a cross-sectional diagram illustrating a ruthenium (Ru) layerdeposited onto the diffusion barrier according to an embodiment of thepresent invention;

FIG. 4 is a cross-sectional diagram illustrating a thick PVD copper (Cu)layer deposited on the Ru layer according to an embodiment of thepresent invention. Note the presence of voiding at the bottom of thevia;

FIG. 5 is a cross-sectional diagram illustrating the structure formedwhen the structure depicted in FIG. 4 is subjected to a reflow annealaccording to an embodiment of the present invention. Note the absence ofvoids in the bottom of the via.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 1-5 are diagrams illustrating an exemplary methodology forfabricating a copper (Cu)-filled via. Advantageously, Cu-filled viasformed using the present techniques have significantly purer copper thanthose formed by electroplating using the prior art, and can befabricated without the use of costly electroplating tools or thenecessity to handle toxic electroplating solutions. To begin theprocess, a dielectric is formed over a substrate.

FIG. 1 is a cross-sectional diagram illustrating a dielectric 120deposited over a substrate 100. Dielectric 120 can comprise any suitabledielectric material, including, but not limited to, one or more ofsilicon dioxide (SiO₂), silicon-carbon-oxygen-hydrogen materials (e.g.,SICOH) and organic polymers, and can be deposited using any suitabledeposition process, such as chemical vapor deposition (CVD), atomiclayer deposition (ALD), evaporation, sputtering or solution basedtechniques, such as spin-on coating, to a thickness of from about 10nanometers (nm) to about 1,000 nm. Substrate 100 generally representsany wiring or contact layer in a single or multilayer wiring array. Anarrow via 101 is then etched into dielectric 120, using any suitableetching process, such as reactive ion etching (RIE). According to theexemplary embodiment shown in FIG. 1, via 101 is formed having a width wof from about 20 nm to about 50 nm.

Next, the via is lined with a diffusion barrier. FIG. 2 is across-sectional diagram illustrating via 101 lined with a diffusionbarrier 202. According to an exemplary embodiment, diffusion barrier 202is made up of two layers. The first layer, a tantalum nitride (TaN)layer 202 a, is deposited onto dielectric 120, to a thickness of fromabout 5 nm to about 15 nm, e.g., from about 8 nm to about 12 nm, so asto line the via. The second layer, a tantalum (Ta) layer 202 b, isdeposited onto TaN layer 202 a to a thickness of from about 5 nm toabout 15 nm, e.g., from about 8 nm to about 12 nm. Diffusion barrier 202prevents the diffusion of Cu (see below) into the dielectric.

A thin ruthenium (Ru) layer is then conformally deposited onto thediffusion barrier. FIG. 3 is a cross-sectional diagram illustrating Rulayer 302 deposited onto diffusion barrier 202 (i.e., onto Ta layer 202b). According to an exemplary embodiment, Ru layer 302 is conformallydeposited onto diffusion barrier 202 to a thickness of from about 1 nmto about 10 nm, e.g., from about 2 nm to about 5 nm using CVD or ALDwith ruthenium carbonyl as a precursor. CVD or ALD ensures uniformconformal coverage of the Ru on the diffusion barrier. Alternatively, Rulayer 302 can be deposited onto diffusion barrier 202 using a sputterdeposition process, provided it is sufficiently conformal for thespecific structure fabricated.

The inclusion of the Ru layer 302 is critical to the invention. The Rulayer 302 acts as a wetting agent for the PVD Cu layer (see below), tofacilitate the reflow of Cu in the reflow anneal (see also below).Accordingly, uniform coverage by the Ru is important to ensuresuccessful reflow.

A thick Cu layer is deposited on the Ru layer via PVD. FIG. 4 is across-sectional diagram illustrating Cu layer 402 deposited on Ru layer302. Note that owing to the limited conformality of the PVD processvoiding is typically present in the via at this stage, as illustrated.According to an exemplary embodiment, the PVD Cu layer 402 is depositedon Ru layer 302 to a thickness of from about 100 nm to about 200 nm,depending upon the exact topographic details of the wiring structure,such as the density, depth and aspect ratio of the vias. This layer isto be contrasted with the thin Cu seed layers of thickness from about 25nm to about 35 nm, used in conjunction with prior art electroplating.

FIG. 5 is a cross sectional diagram illustrating the effect of annealperformed on the structure illustrated in FIG. 4 to reflow the Cu in thestructure and eliminate voids. According to an exemplary embodiment, theanneal is performed at a temperature of from about 150 degrees Celsius(° C.) to about 350° C., e.g., about 250° C., in forming gas (e.g.,hydrogen or a mixture of hydrogen with any gas with which the substratewould not react, e.g., nitrogen or a noble gas). According to anexemplary embodiment, the anneal step can be carried out for a period oftime from about 15 min. to about 60 min., preferably from about 30 min.to about 45 min. Note that the voiding is eliminated by Cu reflow. Alsonote that without the introduction of Ru layer 302 this reflow annealwould not be effective because the seed Cu layer would be deposed upon aTa surface of the diffusion barrier, which is poorly wetted by the Cu.This poor wetting behavior would prevent the Cu from flowing over the Tain order to eliminate the voids as required.

After the reflow anneal is effected, further processing of the wiringstructure can be carried out according to methods known in the priorart.

Although illustrative embodiments of the present invention have beendescribed herein, it is to be understood that the invention is notlimited to those precise embodiments, and that various other changes andmodifications may be made by one skilled in the art without departingfrom the scope of the present invention.

What is claimed is:
 1. A wiring structure comprising: a dielectric layeroverlying a substrate, wherein said dielectric layer includes an openingwithin said dielectric layer; a diffusion barrier lining said opening; aconformal ruthenium layer on said diffusion barrier; and a void-freecopper layer on said ruthenium layer, wherein said copper layer has animpurity element selected from the group consisting of Cl, S, and acombination thereof, wherein the concentration of each impurity elementis less than 20 ppm, and wherein said void-free copper layer is formedby sputter deposition followed by a reflow anneal.
 2. The wiringstructure of claim 1, wherein said opening exposes a topmost surface ofsaid substrate.
 3. The wiring structure of claim 1, wherein said openingincludes a line or a via.
 4. The wiring structure of claim 1, whereinsaid dielectric layer comprises a material selected from the groupconsisting of SiO₂ and SiCOH.
 5. The wiring structure of claim 1,wherein said dielectric layer has a thickness of between 10 nm and 1,000nm.
 6. The wiring structure of claim 1, wherein the copper layercomprises another impurity element selected from the group consisting ofC, O, and a combination thereof, wherein the concentration of each ofsaid another impurity element is less than 40 ppm.
 7. The wiringstructure of claim 1, wherein said conformal ruthenium layer has athickness of from about 1 nanometer to about 10 nanometers.
 8. Thewiring structure of claim 7, wherein the ruthenium layer has a thicknessof from about 2 nanometers to about 5 nanometers.
 9. The wiringstructure of claim 1, wherein said copper layer has a thickness of fromabout 50 nanometers to about 300 nanometers.
 10. The wiring structure ofclaim 9, wherein said copper layer has a thickness of from about 100nanometers to about 200 nanometers.
 11. The wiring structure of claim 1,wherein said diffusion barrier is in direct and continuous contact withan entire exposed sidewall surface of said opening.
 12. The wiringstructure of claim 1, wherein said conformal ruthenium layer is indirect and continuous contact with an entire exposed sidewall surface ofsaid diffusion barrier.
 13. The wiring structure of claim 1, whereinsaid copper layer is in direct and continuous contact with an entireexposed sidewall surface of said ruthenium layer.
 14. The wiringstructure of claim 1, wherein said diffusion barrier comprises: atantalum nitride layer lining said opening; and a tantalum layer on saidtantalum nitride layer.
 15. The wiring structure of claim 14, whereinsaid tantalum nitride layer has a thickness of from about 5 nanometersto about 15 nanometers.
 16. The wiring structure of claim 15, whereinsaid tantalum nitride layer has a thickness of from about 8 nanometersto about 12 nanometers.
 17. The wiring structure of claim 14, whereinsaid tantalum layer has a thickness of from about 5 nanometers to about15 nanometers.
 18. The wiring structure of claim 17, wherein saidtantalum layer has a thickness of from about 8 nanometers to about 12nanometers.
 19. The wiring structure of claim 14, wherein said tantalumnitride layer is in direct physical contact with an entire exposedsidewall surface of said opening, and said tantalum layer is in directphysical contact with an entire exposed sidewall surface of saidtantalum nitride layer.